1. Field of the Invention
The present invention relates to radar. More specifically, the present invention relates to systems and methods for synthetic aperture radar signals.
2. Description of the Related Art
Radar systems are used for a variety of applications. For certain applications, certain types of radar are preferred. For example, for numerous aircraft and satellite applications, synthetic aperture radar (SAR) is frequently used. A synthetic aperture radar images targets on the ground from the air. (See xe2x80x9cRadarxe2x80x9d Microsoft(copyright) Encarta(copyright) Encyclopedia 2000) A SAR system uses the movement of an airplane or satellite carrying it to make the antenna seem much larger than it is. The ability of radar to distinguish between two closely spaced objects depends on the width of the beam that the antenna sends out. The narrower the beam, the better its resolution. A narrow beam requires a large antenna. A SAR system is limited to a relatively small antenna with a wide beam because it must fit on an aircraft or satellite. SAR systems are called synthetic aperture, however, because the antenna appears to be larger than it is. This is because the moving aircraft or satellite allows the SAR system to repeatedly take measurements from different positions. An on-board or ground based processor processes these signals to make it seem as though they came from a large stationary antenna instead of a small moving one. This is the principle by which a SAR achieves its fine resolution in along-track. To achieve a fine resolution in the cross-track, a pulse compression technique is usually employed. This technique enables high signal-to-noise ratio by transmitting long wide-band pulses and achieves a high range resolution by pulse compression to effectively converting long pulses into short ones.
Synthetic aperture radar resolution can be high enough to pick out individual objects as small as automobiles. Typically, an aircraft or satellite equipped with SAR flies past the target object. In inverse synthetic aperture radar, the target moves past the radar antenna.
SAR mapping is typically performed over an angle of 90xc2x0 degrees relative to the velocity vector of the vehicle. An angle of greater or less than 90xc2x0 is referred to as a xe2x80x9csquint anglexe2x80x9d. SAR mapping with a squint angle is often necessary to overcome a flight path restriction in an airborne SAR or an orbit constraint in a space-borne SAR. Processing SAR data collected with a squint angle has been a challenging task due to the constantly changing distance between the radar and a ground target. This so called xe2x80x98range walkxe2x80x99 effect makes the line-like azimuth response of the targets skewed with respect to the along-track direction.
Two decades ago, the main problems associated with processing this data were the cost of large memory required to store the intermediate two-dimensional data and the degraded impulse response due to lack of processing algorithms designed for squint mapping. In recent years, memory cost has been dramatically reduced and high quality processing algorithms have been proposed. Prior approaches for processing squint mode SAR data include the (1) polar format algorithm, (2) range-Doppler algorithm (RDA) and RDA with secondary range compression, (3) range migration algorithm (RMA), and (4) chirp scaling algorithm (CSA).
Unfortunately, there are many disadvantages associated with these prior approaches. For example, the disadvantages associated with polar format algorithm in squint mode processing include (1) a discontinuity of pixel amplitude and phase among image subpatches and (2) considerable complexity associated with the subpatch approach. Further, the range-Doppler algorithm with secondary range compression can handle squint mode data up to certain angle limit. Beyond that limit, the performance of its impulse response is greatly reduced.
In the RMA and CSA approaches, azimuth compression involves two-dimensional Fast Fourier Transforms (FFT). To achieve optimal efficiency, range compression is usually integrated with azimuth compression. This makes it very difficult to efficiently integrate other processes including autofocus, impulse response weighting, and radiometric compensation into the processing chain.
In short, these algorithms have been difficult to optimize with respect to the efficiency of the entire processing chain including autofocus processing, impulse response weighting, coordinate rotation, and radiometric compensation. In addition, due to the complexity of these algorithms, the cost associated with software development and maintenance has been relatively high.
Hence, a need remains in the art for an improved system or method for processing squint-mapped synthetic aperture radar data.
The need in the art is addressed by the system and method for processing squint-mapped synthetic aperture radar data of the present invention. The inventive method includes the steps of effecting range compression of the data; deskewing the data; performing a Fourier transform with respect to the deskewed data; providing a range migration interpolation of the transformed data; effecting a frequency remapping of the range interpolated data; and performing an inverse Fourier transform with respect to the deskewed data.
After the data deskew operation, it might appear that one could perform azimuth compression in the azimuth dimension using a range-Doppler algorithm. However, targets lying in the same range bin but different azimuth angles are associated with different focusing parameters. Therefore, a unique azimuth reference function will not be able to focus all targets in the same range bin. In accordance with the present teachings, this problem is addressed by performing a frequency remapping process for the azimuth spectra after the range migration interpolation and an azimuth reference multiply operation. This removes the quadratic phase term associated with each target such that after inverse azimuth FFT focused impulse response will be achieved.
For some SAR systems where the focusing parameter difference in one range bin is significant such that a single range migration curve cannot approximate all targets, slight degradation would occur on the final impulse response. In accordance with the present teachings, a spatial variant filter, with a small two-dimensional spatial kernel, may be added to provide post impulse compression.